Epigenetic marks are enzyme-mediated chemical modifications of DNA and of its associated chromatin proteins. Although epigenetic marks do not alter the primary sequence of DNA, they do contain heritable information and play key roles in regulating genome function. Such modifications, including cytosine methylation, posttranslational modifications of histone tails and the histone core, and the positioning of nucleosomes (histone octamers wrapped with DNA), influence the transcriptional state and other functional aspects of chromatin. For example, methylation of DNA and certain residues on the histone H3 N-terminal tail, such as H3 lysine 9 (H3K9), are important for transcriptional gene silencing and the formation of heterochromatin. Such marks are essential for the silencing of nongenic sequences, including transposons, pseudogenes, repetitive sequences, and integrated viruses, that become deleterious to cells if expressed and hence activated. Epigenetic gene silencing is also important in developmental phenomena such as imprinting in both plants and mammals, as well as in cell differentiation and reprogramming.
Different pathways involved in epigenetic silencing have been previously described, and include histone deacetylation, H3K27 and H3K9 methylation, H3K4 demethylation, and DNA methylation of promoters. An avenue to achieve DNA methylation is via a phenomenon known as RNA-directed DNA methylation, where non-coding RNAs act to direct methylation of a DNA sequence. In plants, no proteins have been described that link the recognition of a specific DNA sequence with the establishment of an epigenetic state. Thus, plant epigenetic regulators generally cannot be used for epigenetic silencing of specific genes or transgenes in plants.
One solution is to identify or engineer epigenetic regulators that contain sequence-specific zinc finger domains, since zinc fingers were first identified as DNA-binding motifs (Miller et al., 1985), and numerous other variations of them have been characterized. Recent progress has been made that allows the engineering of DNA-binding proteins that specifically recognize any desired DNA sequence. For example, it was recently shown that a three-finger zinc finger protein could be constructed to block the expression of a human oncogene that was transformed into a mouse cell line (Choo and Klug, 1994). However, potential problems to engineering epigenetic regulators that contain an engineered zinc finger domain include ensuring that the engineered protein will have the correct folding to be functional, and ensuring that the fusion of the zinc finger domain to the epigenetic regulator does not interfere with either the DNA-specific binding of the zinc finger domain or the activity of the epigenetic regulator.
Accordingly, a need exists for improved epigenetic regulators that are capable of binding specific DNA sequences, that fold properly, and that retain both the sequence-specific DNA-binding activity and epigenetic gene silencing activity when expressed in plants.